1. Field of the Invention
This invention relates to a dry etching method used for etching of a layer of a copper-based material in the preparation of a semiconductor device. More particularly, it relates to a method for lowering the range of the etching temperature and detecting the etching end point.
2. Description of the Prior Art
In keeping up with the tendency in recent years towards a higher degree of integration and higher performance of semiconductor devices, as with VLSIs or ULSIs, the design rule of metal interconnections tends to be refined to an order of submicrons or even to an order of quarter microns. Up to now, the metal interconnections in semiconductor devices are predominantly formed of aluminum-based materials. However, with the aluminum-based metal interconnections, if the design rule becomes finer than 0.5 .mu.m, not only is the reliability of the interconnection deteriorated due to electromigration or the like, but the aspect ratio becomes as large as 1 to 2 on account of the necessity of maintaining a lower resistance value. This presents difficulties in a series of subsequent process steps such as formation of insulating films or planarization.
Under these circumstances, attention is directed to formation of the interconnection with copper-based metallic materials. Copper has a high resistance to electromigration and a low resistivity of ca. 1.4 .mu..OMEGA.cm which is only about half of that of aluminum. Thus it becomes possible to reduce the thickness of the metal interconnection layer without deterioration in reliability, while the aspect ratio may also be diminished.
However, as to etching of the copper-based metallic materials, its potential is simply searched at the present stage, and many technical problems remain to be solved.
In the first place, since copper is highly susceptible to oxidation, the surface of the copper-based metallic materials is perpetually coated with copper oxide and thereby passivated. The presence of a copper oxide film is inconvenient in that not only is the chemical reaction between the etching gas and copper suppressed to lower the etchrate, but also the layer of the metallic material below the copper oxide film functioning as a mask may not be removed so that residues tend to be produced on a wafer.
On the other hand, it is well-known that copper may scarcely be etched with a halogen-based gas which is extensively utilized for etching other metal interconnection layers. It is because copper halides as reaction products are extremely low in vapor pressure at room temperature. For example, CuCl and CuCl.sub.2 melt at 430.degree. and 620.degree. C., respectively. Thus, unless a wafer is heated to higher temperature for raising the vapor pressure, prompt vaporization of the reaction products can not be expected.
For this reason, in an etching process of eliminating copper in the form of chlorides, the wafer temperature has hitherto been set to approximately 200.degree. to 400.degree. C. For example, according to a technique reported in Japanese Journal of Applied Physics, Vol. 28, No. 6, pages L1070 to L1072, 1989, reactive ion etching of a thin copper film is effected with a SiCl.sub.4 --N.sub.2 mixed gas while the wafer is heated to approximately 250.degree. C.
On the other hand, there is disclosed in JP Patent KOKAI Publication No. 2-83930 (1990) a technique of patterning a layer of a copper-based material by the combination of dry etching and wet etching. Specifically, the layer of the copper-based material is etched by ion milling using a gas mixture of CCl.sub.4 and Ar, after which the wafer is dipped in ethanol to dissolve and eliminate copper hydrochloride reaffixed to the pattern sidewall.
However, many problems remain to be solved with the above described prior-art techniques, which have not yet been put to practical use.
With the technique of heating the wafer for raising the vapor pressure of copper chloride as a reaction product, problems are raised in that, not only an etching mask need to be produced using a highly heat-resistant material, but also the formation of copper oxide tends to be accelerated due to residual oxygen in the etching chamber.
On the other hand, with the technique of using dry etching and wet etching in combination, although the deficiencies due to heating may be obviated, problems are raised in that the number of process steps is increased, a separate treatment vessel for wet etching is necessitated, and in that it is only CuCl.sub.2 that may be dissolved and removed by ethanol, while CuCl.sub.1 which is insoluble in ethanol, can not be removed.
Besides, the method for end point detection at the time of removal of copper as a halide by etching is currently not known, which also is inconvenient if the technique is to be put to practical use.